Lead smelting has in the past been carried out in an ore hearth process but is now most usually conducted by a sintering process. In the hearth process with the furnace in blast at 920.degree. C. to 985.degree. C., ore was charged to float on a bath of molten lead. Air was blown onto the surface whereby lead sulfides were oxidized to lead metal. Alternate layers of coke breeze ensured that lead sulfide oxidized to lead oxide was reduced to lead. Slag forming constitutents of the ore fused and were skimmed from the surface. Molten lead was tapped from the hearth. Only ore concentrates of lead content 70% or higher were considered amenable for such smelting. Typically about 35% of the ore charge became fumed and was recycled.
The sintering process is now the process in general use. Typically pelletized feed is oxidized on a travelling grate. Excess air is drawn through the charge and sulphur dioxide formed is drawn off to inhibit sulfate formation. There is produced on the grate a sinter of lead oxide together with the formation of lead silicates and oxides of zinc, iron and other metals depending on the composition of the ore sintered. The sinter is subsequently conveyed to a blast furnace wherein the oxides are reduced to metals with coke and are separated.
U.S. Pat. No. 3,281,237 proposed a process in which a gas suspended particulate lead sulfide and an oxygen containing gas were introduced concurrently beneath the surface of a pool of molten lead with the object of oxidizing the lead sulfide to molten lead in a continuous single stage operation. The process as described was not developed past the pilot plant stage due among other problems to continued failure of the refractory lining.
U.S. Pat. No. 3,941,587 proposed a process in which a molten bath comprising a metal rich phase and a slag phase is established and maintained beneath a sulphur dioxide gas phase in an elongated tiltable refactory lined sealed furnace. Oxygen is introduced below the surface with a minimum of bath turbulence so as not to interfere with a flow of metal rich and slag phases and a specially arranged oxygen activity gradient towards opposite ends of the near horizontal furnace.
Australian Pat. No. 502,696 relates to a method for the reduction of lead oxide by injection of a mixture of a fuel with air into a bath of molten oxide in a slag, while adding a carbonaceous reducing agent in the form of particles of 1 cm or larger.